s390/qeth: fix off-by-one in RX copybreak check

The RX copybreak is intended as the _max_ value where the frame's data
should be copied. So for frame_len == copybreak, don't build an SG skb.

Fixes: 4a71df50

 ("qeth: new qeth device driver")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>